It is well known in the petroleum refining industry to physically treat hydrocarbons thereby chemically changing their molecular structures and converting less valuable compounds into those which are in demand.
One such conversion process is broadly referred to as "cracking", the thermal decomposition of long-chained hydrocarbon molecules into shorter hydrocarbon molecules having lower boiling points. In its broadest sense, thermocracking is typified by processes in which an unrefined hydrocarbon feed is converted by heating within a reactor vessel to a temperature between 800.degree. and 1500.degree. F. and at a pressure of about 200 to 600 pounds per square inch.
There are two general types of thermocracking processes. The first is known as vapor phase while the second is referred to as liquid or mixed phase. In vapor phase thermocracking, the charge stream is completely vaporized during the high temperature cracking process. In liquid or mixed phase cracking, the charge stream is essentially liquid but there does exist some vaporization and generation of non-condensable gases.
Yet another thermocracking process is known as visbreaking, so named because the process reduces the viscosity of heavy crude oil residues thereby making them more suitable for an inclusion into, for example, fuel oils. In visbreaking, a heavy crude oil residue is passed through a thermal reactor or heat exchanger comprising a series of tubes which are subjected to high temperature heat exchange. The heavy crude oil residue is heated above 700.degree. F. to about 900.degree. F. or higher and held at that temperature within the reactor tubes for a period of time sufficient to produce the desired amount of cracking. A gas oil distillant is continually produced and removed as it is generated.
Regardless of the type of thermocracking, all cracking processes deposit or accumulate carbon or coke on the reactor surfaces as well as in apparatus downstream of the reactor. For example, impurities in the charge material such as salts or metal compounds may become deposited on the heat exchanger reactor walls. The deposits gradually increase in thickness and eventually cause a reduction in cracking efficiency of the heat exchanger through the loss of heat transfer. In addition, the excessive carbon deposits slowly insulate the reactor or heat exchanger surfaces as they build up requiring greater and greater quantities of heat to maintain cracking temperatures. This additional heat only further exasperates the problem by producing even more carbon deposits on the reactor surfaces. Eventually, the reactor must be shut down and the deposits are manually removed.
Numerous prior art attempts have been made to prevent the build-up of carbon on the interior walls of a parallel pass reactor or heat exchanger. Many parallel pass reactors or heat exchangers incorporate abrasive, granular media which fluidizes in the process liquid as it travels through the reactor. The granular media impact against the interior tube walls and abrade the carbon as it is formed. The abraded carbon particles are then removed from the reactor during processing without the need for reactor shutdown. For example, U.S. Pat. No. 4,427,053, to Klaren discloses a heat exchanger of the type in which a liquid is passed through a vertical riser tube connected at its upper and lower end to upper and lower tanks respectively and in which a granular mass is present. As the granules are fluidized by the liquid in the reactor, they travel upwardly through the tubes and have a scouring and cleansing effect upon the tube walls.
Although the Klaren device and similar apparatus have addressed the problem of excessive carbon build up, such reactors are not entirely satisfactory when used in connection with a thermocracking or visbreaking operation. Since both thermocracking and visbreaking require heating of the hydrocarbon oil above 700.degree. F., coke and carbon is readily formed as a byproduct. While reactors and heat exchangers which incorporate abrasive particles for circulation do in fact continually clean the interior walls of the riser tubes of deposits, once the visbroken or thermocracked liquids and vapor exit the riser tubes and enter the head portion of the reactor, they continue to crack and deposit carbon within the upper chamber of the reactor and downstream of the reactor. Eventhough the abrasive particles are effective in cleaning the interior of the heat exchanger riser tubes, once outside the narrow tubes the abrasive media is ineffective. Consequently, the thermocracking liquid vapors and gases continue to crack and deposit carbon. Thus, even reactors containing abrasive particles must be shut down for periodic cleaning if the reactors are being used in thermocracking or visbreaking operations. Previously, there has been no effective way to control such carbon deposition within a heat exchanger or reactor employed in such processes.
A need has therefore existed within the art to provide a cracking and visbreaking heat exchanger reactor which minimizes carbon deposition to the reactor as well as downstream of the reactor and thereby ensures long periods of operation without the need for reactor shutdown or periodic cleaning.